DER-111

3.15 W General Purpose Adapter

October 26, 2005

Page 6 of 23

Power Integrations

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5

Circuit Description

This circuit is configured as a Flyback operating in discontinuous conduction mode. The

low standby consumption is achieved by using a high gain opto-coupler, using a bias

winding that provides about 8 V during no-load, and by designing a low-capacitance

transformer.

5.1

Input Rectification, Bulk Capacitance and EMI Filtering

AC input power is rectified by a full bridge, consisting of D1 through D4. The rectified DC

is then filtered by the bulk storage capacitors C1 and C2. Inductor L1 and Ferrite bead

L2 separate C1 and C2 from each other. L1, C1 and C2 form a pi (

π) filter, which

attenuates conducted differential-mode EMI noise. Fusible resistor RF1 has multiple

functions. It is a fuse, an in-rush current limiting device, a final low pass filter stage (with

C1) for conducted EMI attenuation and an initial stage of input surge voltage attenuation.

5.2

Primary DRAIN Voltage Clamp Circuit

The DRAIN voltage clamp circuit is comprised of Zener diode VR1, R1 and diode D5. D5

and VR1 clamp the amplitude of the voltage spike that the transformer leakage

inductance generates, at switch turn-off, to keep it beneath the device’s maximum DRAIN

to SOURCE voltage rating (700 V). R1 damps the high frequency ringing caused by

leakage inductance, which improves the conducted EMI performance of the circuit. The

has been kept low (70 V) to minimize the power dissipation in the clamp circuit.

5.3

Auxiliary Bias Supply

The auxiliary bias supply circuit is made up of the primary-side transformer bias winding,

diode D6 and capacitor C5. D6 rectifies the output of the winding and C5 filters it. The

winding was given just enough turns so that its minimum output voltage stays at 7V at no-

load to minimize power consumption. C4 is the standard BP pin decoupling capacitor,

which should always be a 50 V 0.1

µF ceramic capacitor that is located close to the IC. R3

and C9 are compensation network, along with the Linkswitch Control pin resistance, form

a pole and a zero to stabilize the control loop.

5.4

Output Rectification and Filtering

Output rectification and filtering are accomplished by Schottky diode D7, capacitors C6

and C7. D7 rectifies the output of the transformer, T1.

5.5

Output Voltage Sensing and Feedback

Resistors R4, Zener diode VR2 and opto-isolator U2 sense the output voltage and

feedback to the LinkSwitch. C10 provides power for the LinkSwitch during the time of

power up, it also acts as a soft start capacitor.